1. Field of the Invention
The present invention relates to a backlight power supplying circuit for supplying power to a backlight. In particular, the invention relates to a backlight power supplying circuit having a function of protecting the circuit against voltage drop, and to a liquid crystal television (TV) with use of the backlight power supplying circuit.
2. Description of the Related Art
FIG. 6 shows a conventional backlight power supplying circuit. As shown in FIG. 6, a backlight power supplying circuit 1 includes a main power supply circuit 3, a main transformer 4, an inverter circuit 5, a transformer 6, a control unit 8, and a determining circuit 9. The main power supply circuit 3 rectifies and reduces a voltage of a commercially available 100V AC power supply, which is input from a plug socket 2, and the voltage is divided at the main transformer 4 to thereby supply power to the inverter circuit 5. Further, the main transformer 4 supplies power to various circuits other than the inverter circuit 5. The inverter circuit 5 is intended to increase the voltage divided through the main transformer 4 due to the switching action of a MOSFET etc. up to an output voltage level necessary for the backlight 7. In this case, such MOSFETs execute a switching function behind the control of the inverter controlling IC 5a incorporated into the inverter circuit 5.
The control unit 8 controls driving of the main power supply circuit 3 and the inverter circuit 5. In response to an instruction to turn the power on the backlight 7 by means of a remote controller (not shown), the control unit 8 starts driving the main power supply circuit 3 and sends a signal BL_on that triggers the inverter controlling IC 5a to execute the switching function. As a result, the inverter circuit 5 boosts the power supply voltage input through the main transformer 4 and outputs the boosted voltage to the backlight 7 through the transformer 6. Further, a secondary coil of the transformer 6 makes determination about the output voltage for the backlight by means of the determining circuit 9. If the determining circuit 9 detects any abnormality in output voltage, a feedback signal is sent to the control unit 8. The control unit 8 receives the feedback signal to control the power supply of the main power supply circuit 3.
However, if a voltage of the commercially available power supply, which is supplied from the plug socket 2, drops due to temporary blackout (momentary blackout), a power supply voltage input from the main power supply circuit 3 through the transformer 4 is, of course, lowered, with the result that a voltage level of the signal BL_on lowers. At this time, the inverter controlling IC 5a stops driving the inverter circuit 5 without recognizing the signal BL_on. The inverter controlling IC includes, as a protective functional unit, a protective circuit 5a1 that stops driving the inverter circuit 5 when a voltage level of the signal BL_on is lowered. Since the output voltage generated with the inverter circuit 5 is abnormally as high as 1000 V, the protective circuit temporarily cuts off the supply of the signal BL_on from the control unit 8 (cuts off a main power supply) to thereby prevent the backlight 7 from being damaged due to malfunction of the inverter circuit 5. Hence, upon such temporary blackout, a user needs to turn on the main power supply again to redrive the liquid crystal TV.
Japanese Unexamined Patent Application Publication No. 2004-355865 discloses a technique using a switch and control circuit for resetting a latch state if a controlling IC for controlling a main power supply circuit is shifted to the latch state due to temporary blackout.
The technique as disclosed in Japanese Unexamined Patent Application Publication No. 2004-355865 has the following problem. That is, the publication does not contain any reference to an inverter circuit supplied with power from the main power supply circuit through a main transformer. Thus, in the configuration including a controlling IC on the inverter side as well as on the main power supply circuit side, it is necessary to take a countermeasure against the temporary blackout for the inverter controlling IC on the inverter side.